You may have a few fungus gnats but it's not all that bad - and they certainly aren't causing the discoloration on the leaves... You can kill fungus gnats with Mosquito Bits - you just follow the directions and use the water for your plant and the fungus gnats will be history (with this minor infestation, one application will suffice).
No, this lady is desperate for some good NPK nutes right about now and I don't see that you're giving her anything. Get her going on some good grow nutes plus a little bit of calmag and she should turn a better shade of green AND take care of the phosphorus deficiency it looks like she has...
And btw, fungus gnats like to live in very damp soil so this is telling me you're overwatering her.... weed needs a definite wet/dry cycle in order to grow properly - make sure your soil is completely dry before giving her the next in your water/nute/water/nute routine....
But for now, get her some food! Good luck!